1. Field of the Invention
This invention relates generally to processes and electronic oscillating circuits, and, more particularly, to processes and oscillating circuits able to correct frequency variations in oscillating circuits caused by changes in temperature and other environmental conditions, and able to also correct frequency variations caused by environmental factors in non-crystal periodic sources.
2. Description of the Related Art
Oscillating circuits play a central and increasingly important role in digital and analog electronic systems. Digital devices require precise system timing, a function provided by oscillators and similar timing sources. Telecommunication and data transmission systems, which have analog and digital components, likewise rely on oscillators for modulation, demodulation, system clocking, and other functions.
A standard choice for a highly stable frequency source in such applications is a crystal-based oscillator or resonator. (Atomic frequency standards, while highly accurate, are undesirable in most such applications because of cost and packaging considerations.) While stable in comparison with non-crystal based resonating circuits, crystal oscillators and resonators nevertheless exhibit a degree of frequency instability owing to a crystal""s inherent frequency response to temperature changes and to other environmentally influenced factors such as aging. See the paper titled Frequency-Temperature-Angle Characteristics of AT-Type Resonaters Made of Natural and Synthetic Quartz, Rudolf Bechmann, Proceedings of the IRE, November, 1956, pp. 1600-1607.
Current practice to correct such frequency instabilities follows two basic approaches. The first is represented by temperature compensated crystal oscillators (TCXOs) and digitally compensated crystal oscillators (DCXOs). In these designs, circuit elements sense the ambient temperature (which reflects the temperature of the crystal element) and the reactance loop of the oscillator circuit is adjusted to correct the frequency output. A DCXO differs from a TCXO primarily in the method employed to acquire temperature-related information used to compensate the oscillator circuit. A DCXO typically uses a temperature sensor, microprocessor and EPROM to acquire and store compensation data; a TCXO""s compensation network uses analog devices solely, such as thermistors.
The second approach achieves frequency control by simply maintaining the oscillator""s crystal element at a constant ambient temperature during operation, thus eliminating temperature as a cause of frequency variation. This approach is taken by the oven compensated crystal by oscillator (OCXO).
The relative success of these approaches varies. An OCXO can be manufactured which is significantly more accurate than a TCXO or DCXO. TCXO and DCXO oscillators are typically offered in the marketplace with accuracies from 5 ppm to 0.5 ppm. OCXO oscillators can be manufactured with accuracies from 0.5 ppm to 0.005 ppm. There is some overlap in accuracy between low end OCXOs and high end TCXOs and DCXOs. There are disadvantages with the OCXO relative to the TCXO and the DCXO, namely that it requires more power to operate, generates much waste heat, requires a substantial warm-up time, and occupies a bulky package. TCXO and DCXO oscillators have their own limitations, including relatively complex compensation networks (e.g., number of thermistors and other circuit elements to adjust the reactance loop) as well as the need to begin with a well-tuned, precise oscillator circuit and crystal element. These requirements make fabrication of TCXO and DCXO devices relatively elaborate and costly, although manufacture is generally less costly for TCXO and DCXO devices than for OCXOs.
Recent exemplars of contemporary practice include Watanabe et al. (U.S. Pat. No. 5,548,252, Digital Temperature Compensated Crystal Oscillator, Aug. 20, 1996). This oscillator uses a digital temperature compensated crystal oscillator (DTCXO) system with a memory that stores temperature compensation data received. Post et al. (U.S. Pat. No. 5,525,936, Temperature-Compensated Oscillated Circuit, Jun. 11, 1996), attempts to provide a temperature compensated oscillator circuit constructed with an oscillator controlled by a processor. The output frequency of the oscillator, or an external reference frequency, is used as a reference signal in conjunction with a dual mode oscillator that can be switched to provide temperature-dependent fundamental and third harmonic frequencies.
Connell et al. (U.S. Pat. No. 5,481,229, Low Power Temperature Compensated Crystal Oscillator, Jan. 2, 1996), shows a temperature compensated crystal oscillator constructed with a crystal oscillator circuit, a voltage controlled reactance element, a temperature compensation network, and a programmable DC-DC converter network having an output connected to the voltage controlled reactance element, or to the temperature compensation network, or both. Ishizaki et al. (U.S. Pat. No. 5,473,289, Temperature Compensated Crystal Oscillator, Dec. 5, 1995) has a temperature compensated crystal oscillator with an oscillation circuit, a temperature detecting circuit, and a control signal generating circuit, which is used as a reference frequency oscillator in a mobile communication device, such as a car telephone, a portable telephone, and a cordless telephone, a satellite communication device, and the like. Pucci et al. (U.S. Pat. No. 5,459,436, Temperature Compensated Crystal Oscillator With Disable, Oct. 17, 1995) discusses a temperature compensated crystal oscillator (TCXO) with a disable feature adapted to disable or enable temperature compensation. The TCXO includes a crystal oscillator and a temperature compensation circuit.
Our study of contemporary practice leads us to conclude that contemporary practice fails to provide an oscillating circuit capable of effectively generating a periodic signal exhibiting a stable period in the presence of frequency fluctuations caused in the circuit by the effect of temperature changes and other changing environmental conditions such as crystal aging on the crystal element.
Accordingly, it is an object of the present invention to provide a circuit and process for improved frequency correction for an oscillating circuit.
It is another object to provide a circuit and process able to reduce frequency variation without adjustment to the circuit""s reference clock.
It is yet another object to provide a circuit and process for correcting variations in clock frequency by adjusting a digitally synthesized output frequency.
It is still another object to provide a circuit and a process for generating a periodic signal exhibiting a stable period while using a low cost, low precision reference clock.
It is still yet another object to provide a circuit and process for generating an output frequency comparable in stability to that offered by an OCXO while using a low cost, low precision reference clock.
It is a further object to provide an oscillator circuit and process capable of achieving a level of frequency stability greater than that of a typical TCXO and DCXO, without the elaborate and finely tuned design required by a precise TCXO and DCXO, and which is easier and less costly to manufacture.
It is also an object to provide an improved digital synthesizing process and device for generating periodic frequencies with a stability and accuracy greater than that exhibited by the a reference clock driving the device.
It is a yet further object to provide an oscillator circuit and process capable of achieving a level of frequency stability comparable to that offered by a standard OCXO, without being burdened with the OCXO""s disadvantages, including high power consumption, significant warm-up time, heat loss, a large package, and high manufacturing cost.
These and other objects are achieved through the use of a direct digital synthesizer (DDS) in a frequency correction circuit. The synthesizer generates a synthetic output frequency. It is driven by a reference clock and is therefore affected by the clock""s frequency variations caused by temperature and other conditions. The output frequency of the synthesizer, however, can be controlled (and therefore adjusted) with a high degree of precision through a digital instruction is programmed by a microcontroller or microprocessor. In response the direct digital synthesizer produces an output frequency as specified by the digital instruction. Using a direct digital synthesizer enables the frequency correction circuit to correct frequency variations continuously and very precisely. This permits adjustment of frequency for non-temperature environmental factors such as aging and acceleration, and is capable of compensating for frequency variations in non-crystal resonator periodic sources such as ceramic resonators or satellite-generated periodic sources.
There are two basic modes of operation of a device expressing a preferred embodiment of this invention: programming and operation. The purpose of programming is to evaluate the temperature-caused frequency variations experienced while running the device over its specified temperature range, and to generate frequency correction instructions to correct the variations, which instructions are then stored in a data storage area for future reference during normal operation of the device. There are two frequency sources in the device. One is a clock reference frequency which is required to clock the device""s direct digital synthesizer and the device""s digital microcontroller. The second is a frequency generated by the direct digital synthesizer. This is the xe2x80x9coutput frequencyxe2x80x9d which is adjusted by the frequency correction scheme of the present invention. No frequency compensation of the reference clock is attempted.
The device is programmed during the manufacturing process. The device is operated in a test oven at temperatures sequenced by one-half of a degree or other temperature division throughout the specified temperature range of the device. A temperature sensor senses the device temperature at each temperature point. The temperature is encoded as binary data and is accessed by a programming computer either directly or through a microcontroller in the device.
At each temperature point the programming computer reads the actual output frequency of a direct digital synthesizer (DDS) via a frequency counter and compares it with the target output frequency. When there is a discrepancy between actual and target output frequencies, the programming computer derives a frequency correction instruction which it issues to the direct digital synthesizer. The synthesizer responds by adjusting its output frequency as specified in the instruction. The programming computer continues to read the output frequency of the direct digital synthesizer at regular intervals at the given temperature point and continues to issue frequency correction instructions until the output frequency of the synthesizer matches the target output frequency. Then the programming computer stores the temperature data and the last frequency correction instruction in the data storage area (EEPROM). The measured temperature data serves as an index to the frequency correction instruction for later reference during normal operation of the device.
When the frequency correction instruction has been stored for a given temperature point, the oven temperature is changed to the next temperature point in the sequence. The process of reading the output frequency of the synthesizer, generating a frequency correction instruction, and storing the last instruction in the data storage device, is repeated for each temperature point in the specified temperature range of the device. Programming is completed when frequency correction instructions have been stored for each temperature point in the specified range. Thus, when the device is programmed, frequency correction instructions exist for each temperature point within the specified operating temperature range of the device.
In the operating mode, the temperature sensor senses the ambient temperature which is periodically read by the device""s microcontroller. The microcontroller reads the frequency correction instruction corresponding to the ambient temperature from the lookup table in the EEPROM and issues that instruction to the synthesizer. The synthesizer produces the output frequency specified by the instruction. The process of temperature monitoring and adjustment of the output frequency from the synthesizer is repeated at regular intervals, e.g., each quarter of a second, while the device is operating. In this manner the output frequency is continuously adjusted. As noted, the reference clock""s frequency is not adjusted.